I usually carry every fly I own, even if I'm going light, I still carry a small lumbar pack with three fly boxes in it, plus a couple in my vest, plus a small streamer box. I'm deathly afraid of NHTRF. (Not having the right fly) when I'm 3 miles from my car. Sometimes you might run into a spinner fall, or a flying ant attack, or nice fish slurping midges in a big quiet pool. You may catch 3 or 4 fish with generic patterns, but if you have something that matches what's happening on the water you may catch 10 or 12. That being said, 8 times out of 10 I will fish a yellow neversink with a BHPT or a green weenie, so you could carry 1 box with a dozen flies and be fine, but what's the fun in that? I already told my wife that I need to tie at least 50 patterns in 3 different sizes in light, medium, and dark!
Tad